This invention relates to pressure sensors and more particularly to extremely thin pressure sensors of the so-called electrostatic capacitor type.
Japanese Patent Publication Tokkai 7-55615 disclosed a pressure sensor of this type, having an elastic planar dielectric member between the electrodes of a capacitor element. As a pressure is applied to such a pressure sensor and the dielectric member undergoes an elastic deformation, the distance of separation between the electrodes changes, causing the electrostatic capacitance of the capacitor to also change. A pressure change can thus be detected by measuring this change in electrostatic capacitance.
Prior art pressure sensors of this type are required to have a certain substantial thickness, however, because the detection of pressure depends on the elastic deformation of the dielectric member in the direction of its thickness. As the dielectric member is deformed, furthermore, the electrode on the pressure-receiving side is also deformed. As material fatigue accumulates, the useful lifetime of the sensor is adversely affected.
Moreover, dielectric constants of elastic dielectric materials tend to vary with temperature, and their characteristics are unstable. If means for eliminating the effects of temperature variations must be introduced, the sensor structure becomes complicated and adversely affects the production cost.
It is therefore an object of this invention, in view of the problems of the prior art technology described above, to provide extremely thin pressure sensors which are simple in structure but high in durability.
Pressure sensors embodying this invention may be characterized as comprising a pressure-sensitive membrane, of which the dielectric constant changes with pressure, stacked and sandwiched between and in contact with a pair of mutually oppositely facing electrodes with a constant separation. Extremely thin pressure sensors can be obtained according to this invention since elastic deformation in the direction of thickness is no longer involved. Since there is no part which undergoes any elastic deformation, furthermore, there is no problem of material fatigue and hence the durability of the sensor improves. Moreover, there is no need to use an elastic dielectric material which is sensitive to external temperature variations. Thus, this invention can provide pressure sensors with a simple structure which are easy to manufacture.
One of the pair of electrodes may be placed on a surface of a ceramic base plate or a flexible film in a layered formation. Thus, this invention can provide pressure sensors which are easy to use, depending on circumstances.